Commercial devices which effectively handle suspensions, such as paper pulp, at medium consistency, that is at about 6-15% solids consistency, are known. It is also known that air or, more generally gas, if present in the fiber suspensions causes problems in almost all process stages in the pulp and paper industry. When pulp is pumped, mixed, screened, washed or otherwise handled without excess gas significant savings in equipment, power consumption and the like can be achieved. For instance, one device which has been particularly successful in allowing handling of gas-containing medium consistency fiber suspensions is a fluidizing centrifugal pump which simultaneously pumps and degasses the suspension. Typically, such pumps utilize a separate vacuum pump, piping from the centrifugal pump to the vacuum pump, a separate motor and motor mount for the vacuum pump, etc., in order to exhaust the gas which has been separated from the suspension so that the suspension may be effectively pumped by the pump impeller.
U.S. Pat. No. 3,230,890 discloses a centrifugal pump for removing gas from low consistency suspensions or from water having either a built-in vacuum pump or an external vacuum pump.
A fluidizing centrifugal pump having a built-in vacuum pump is disclosed in U.S. Pat. No. 4,776,758. FIG. 1 illustrates the prior art centrifugal pump, with the volute being omitted, provided with a vacuum pump on the same shaft as impeller in accordance with U.S. Pat. No. 4,776,758. The characteristic features of the prior art pumps on the market today and which have not, however, proven to be successful due to some shortcoming in the structure thereof, are disclosed in detail in the following. The pump has a fluidizing impeller 12 rotating in an ordinary medium consistency pump housing. The impeller 12 has through bores 14 for allowing the air accumulated at the front side of the impeller 12 to be drawn by means of the vacuum pump 10 to the back side of the impeller 12. The impeller has also so-called back vanes 16 on the back side thereof for separating the fiber suspension from the medium being drawn through the openings 14 in the impeller plate 18. The main purpose of the back vanes 16 is to pump the fiber suspension back to the pump volute and thus prevent the fibers from entering the vacuum pump 10, as the risk of damaging the vacuum pump 10 rises dramatically if the fibers are allowed to enter the vacuum pump 10. The vacuum pump 10 is a so-called liquid ring pump which has been arranged on the pump shaft 20 behind an intermediate plate 22 in which only a narrow ring-shaped duct 24 is provided which duct surrounds the shaft 20 or the impeller extension 26 for allowing the gas to flow towards the vacuum pump. The intermediate plate 22 is also provided with a ring-shaped channel 28 and a narrow duct 30 leading thereto for introducing make-up air to the vacuum pump while the pump is running. The duct 30 is connected via channel 32 to a vacuum regulating valve (not shown). The vacuum pump housing 34 is provided with a conduit 36 for feeding liquid to the liquid ring pump 10 for maintaining the amount of liquid substantially constant therein. Conduit 36 is connected to the outer, eccentric circumference 38 of the liquid ring pump 10. In other words, the conduit 36 leads exclusively and directly to the liquid ring. The suction opening for the liquid ring pump 10 is provided, naturally, on the side of the centrifugal impeller 12. The discharge channel (not shown) for the gas to be removed from the pump 10 is arranged at the opposite side of the vacuum pump 10, i.e. on the back side of the vacuum pump relative to the centrifugal impeller 12.
Various problems have, however, been encountered with the pump in operation today. For example, the air removal capacity has been significantly lower than required, i.e. the vacuum created has not reached a sufficiently high level. Also, the discharge pressure of the vacuum pump has been found to be too low. In some cases, the material discharged from the vacuum pump, a mixture containing mainly gas but also some fibers, has been introduced into the top portion of a mass tower to recover the fibers. If, however, the discharge pressure of the vacuum pump is too low the pumped material cannot be conveyed to the top of the mass tower, and an additional pump must be installed for that purpose. Also, the open annular volume in the intermediate plate 22 of prior art pump has a tendency to become clogged by the fibers.
In the prior art pump the axial gap 40 between the vanes 42 of the vacuum pump 10 and the axially adjacent walls 44 of the vacuum pump housing are not adjustable but are positioned at a distance or clearance of about 0.4 mm. The reasons for such relatively large clearance is the fact that there are a number of factors which render it is impossible to further decrease the clearance 40 as the various components of the pump are installed on the shaft or around the shaft starting from the drive end 46 of the shaft. Thus, the dimensions of the components effect the clearance 40. The result of too wide a clearance is, of course, excess leakage and an insufficient vacuum. Another reason for the wide clearance 40 may also be the fact that the shaft 20 of the pump tends to flex somewhat during operation creating the risk of mechanical contact between the vacuum pump vanes and the housing walls 44. Thus, the large clearance 40 has been provided intentionally to ensure long lasting operation of the pump.
The pump in accordance with the present invention is designed to eliminate most or all of the above problems. Accordingly, the pump of the present invention provides means for adjusting the axial position of the vacuum pump rotor relative to the front and rear wall of the vacuum pump chamber thereby providing significantly smaller operational clearances or distances therebetween. This may be achieved by either adjusting the axial position of the rotor with respect to the shaft, for example, by the addition of shims between respective shoulders of the vacuum pump rotor and shaft. The relative axial position of the vacuum pump rotor with respect to the vacuum pump chamber may also be optimized by adjusting the axial position of the shaft with respect to the vacuum pump chamber and the centrifugal pump body, in which case the vacuum pump rotor is fixedly attached to the shaft. Finally, the relative axial position of the vacuum pump rotor and the vacuum pump chamber is optimized by adjusting the vacuum pump chamber with respect to the rotor and the centrifugal pump body, for example, by adjustment screws as is further described in detail below.
In addition, ports for the admission of make-up air for the control of the vacuum pump may be provided at the rear wall of the vacuum pump and means are provided to introduce a liquid into the pump for flushing the vacuum pump and conduits leading thereto and maintaining the same free from fibers which otherwise tend to block the flow path of the pump.
Axial clearances between the vacuum pump rotor and the vacuum pump chamber walls may also be adjusted by providing a rotor with rotor blades which are slightly inwardly tapered in radial direction or wherein the side walls of the vacuum chamber are slightly outwardly tapered in radial direction relative to the shaft to account for the slight bending or flexing of the shaft during operation of the vacuum pump.
The vacuum pump may also be designed so that the gas inlet port and the gas outlet port are on the same site of the pump in the intermediate wall and wherein the vacuum pump rotor central portion is conically tapered toward the gas outlet of the pump so as to prevent the formation of a gas pocket around the rotor central portion.
The centrifugal pump impeller may also be provided with a rotor having fluidizing blades either within the pump inlet or entirely outside the pump inlet or any combination thereof.